In electrical equipment having multiple modules or units that are interconnected, it is desirable to provide a keying system that uniquely defines plug and receptacle pairs that are to be mated. There are many such systems in use in the industry. Such systems, however, are typically molded into the connector housings and require considerable space. For example, U.S. Pat. No. 4,778,411 discloses mating electrical connector housings having hexagonal nests formed therein containing hexagonal shaped keys that are positionable in any of six unique angular positions. Each housing has a key that is in axial alignment with a corresponding key in the other housing. Mating of the two housings is prevented unless the two keys are in mutually complimentary angular positions. The shanks of the keys include an undercut that mates with an inwardly facing bead on the interior of the nest and a split end to provide a snap action when the keys are pressed into their seats. The mating ends of the keys are quite long and have tapered ends for lead in and guiding as the connector halves are mated. The large physical size of these keys, resulting from the long tapered ends and the retaining structure, makes this arrangement unsuitable for use with low profile, high density connectors where the available vertical space is limited.
What is needed is a connector keying arrangement that effectively prevents inadvertent mating of the contacts of a plug with the contacts of a receptacle in a low profile connector. Additionally, the keying arrangement should not require that the connector be made larger to accommodate the keys or to require additional space on the circuit board.